The present invention is directed to an assembly and process for forming flexible cutting line for use in rotary trimmers which is comprised of two monofilament strands joined by a severable bond wherein each strand defines a plurality of raised elongated cutting edges.
Flexible line rotary trimmers are used for cutting vegetation such as grass and weeds, particularly along walks, fences and flower beds and around trees. These devices comprise a motor driven rotary head which carries one or more lengths of monofilament line mounted on a spool within a housing. Extended end portions of each line project from the spool through guides in the side of the housing. As the head rotates at high speed, the end portions of the lines are caused to project outwardly from the housing by the centrifugal forces acting thereon and function as cutting blades. The majority of trimmer heads presently in use employ two separate monofilament nylon lines which are both mounted on a common spool and project from the spool and housing through diametrically opposed guides in the trimmer head housing.
The spool which carries the line is mounted within the housing such that it rotates with the housing during use but can be selectively rotated relative the housing to pay out additional line when the projecting end portions of the line become worn or severed. Because these heads typically employ two separate cutting lines, and occasionally three or four such lines, care must be taken in winding the lines about the common spool to prevent the lines from crossing over one another or otherwise tangling within the housing. If the lines become tangled within the housing, additional line cannot be payed out during use or even pulled from the head without having to disassemble the head. This problem is particularly acute in fully automatic and bump-feed heads wherein even the slightest tangle can interfere with the proper indexing and paying out of the line. In addition to interfering with the proper line feeding mechanisms of the flexible trimmer heads, internal tangles can also cause balance and vibration problems which make the trimmer more difficult to use.
To solve the problem of line tangle, monofilament line has been developed which comprises two strands secured together along their adjacent lengths by a readily severable weld. The resulting double strand line is then simply wound about the spool and the end portions separated along their weld so that the end portions of the separated strands can be fed out through the opposed guides. Bonding the two strands together in this manner along their entire lengths prevents tangling of the strands within the housing and, if the strands are properly joined, allows the strands to be readily separated for the feeding of new line through the opposed guides. Such a double or dual-strand line and methods and apparatures for manufacturing such line are disclosed in applicant's pending applications, Ser. No. 08/597,178, filed Feb. 6, 1996, and Ser. No. 08/782,333 filed Jan. 13, 1997.
In an effort to improve the cutting ability of monofilament line, the line has been extruded so as to define a plurality of raised cutting edges extending longitudinally along the line. Different cross-sectional configurations such as squares, diamonds and star shapes, have been developed to provide these multiple cutting edges. In certain designs, two sets of alternating elongated edges are provided. The first set protrudes beyond the second set and the recessed edges in the second set are disposed between the edges in the first set. Accordingly, when the more outwardly protruding edges in the first set become worn down and rounded with use, the second set of sharp edges will come into contact with the vegetation and thus effectively provide a set of fresh, sharp cutting edges with which to trim the vegetation.
Edged cutting line is becoming increasingly popular. However, such line is subject to the entanglement problems discussed above and a method for economically forming a suitable dual line with edged strands has not heretofore been developed. The first dual-strand lines were formed by securing two strands together along their adjacent lengths by a suitable adhesive. Such line, however, was found to be excessively expensive to manufacture and the strength of the adhesive bond between the two strands was inconsistent and caused premature separation of the strands. As a result, such a process is equally ill-suited for use in forming a dual-strand edged line.
In the forming process previously developed by applicant, which is the subject of the above identified patent applications, the problems with adhesively formed dual-strand lines were overcome through the use of a novel extrusion process. One or more pairs of nylon strands are extruded with the strands in each pair being in proximate disposition. The strands are directed from the extrusion die into a cooling quench bath where the two strands in each pair are brought together on a collecting guide in a side-by-side abutting disposition to initiate the forming of a continuous weld therebetween. The pairs of adjoining strands are then pulled through the quench bath as the strands in each pair begin to crystallize and bond together to form a plurality of pairs of joined monofilament strands. It has been found, however, that while that process provides an excellent weld for monofilament strands having a circular cross-section, it is not particularly well-suited for bonding together pairs of nylon polymer edged strands having a plurality of raised cutting edges extending therealong.
In the above-discussed process, the two strands are brought together in a side-by-side disposition within the cooling quench bath to sufficiently crystallize the strands prior to contact to prevent the two strands from permanently fusing together upon contact. As the pairs of closely spaced molten strands extend downwardly from the extrusion die to the collecting guide where the strands in each pair are brought into contact below the surface of the cooling water, the individual strands tend to twist. While this is of little consequence when dealing with strands which are circular in cross-section, if the strands define elongated projecting ribs and one or both of the strands becomes slightly twisted, the ribs on the two strands will not align when brought together but cross over one another. The result will be an inconsistent weld and an unsatisfactory product. While it may be possible to avoid such twisting with extensive care in the handling of the strands within the cooling bath, the care required would not be compatible with an economically viable manufacturing process and twisting might still occur. It was discovered, however, that proper alignment of the strands is not the only problem in attempting to extrude such a product.
As noted above, edged cutting line can be formed in numerous cross-sectional configurations. Depending on their shape, a pair of edged strands could be joined together in a dual-line configuration along a pair of abutting flat surfaces, along abutting edges or a pair of curved surfaces, or possibly along combinations of such surfaces. If the strands could be joined along a rounded surface, like a conventional dual-strand line, applicant's previously discussed extrusion process could be employed to produce the line, provided the line twisting problem could be solved. However, when joining the strands together along a curved surface, the available remaining space for the projecting edges is quite limited and thus the number of projecting edges which could be formed on each strand would be very limited. In addition, it is generally preferred to provide a uniform distribution of the cutting edges about the strands. As a result, the more popular edged lines are square in cross-section or of a diamond or of a star-like shape so as to define multiple raised edges evenly spaced about the outer surface of the line. Attempting to bond a pair of edged strands together along abutting flat surfaces, however, would result in inseparable fusion, even though the contact between the strands occurred in the quench bath, due to the large area of surface contact between the two molten strands. Attempts to bond edged strands along edge-to-edge contact using the aforesaid extrusion process would also prove unsuccessful. The contact area between the two strands was found to be too small to provide a suitable weld unless the edges were relatively rounded. There is a perception, however, that sharp edges provide superior vegetation cutting. If the two edged strands were pressed together along their abutting edges in an effort to strengthen the weld, there was a substantial risk that inseparable fusion would result.
Efforts were then made using applicant's previously developed extrusion process to form pairs of star-type edged strands in a dual line configuration along a pair of abutting edges or star points, as opposed to joining the strands in single edge-to-edge contact. The resulting configuration would comprise two isolated welds extending along the points of contact, separated by the recessed wall surfaces of the strands. The total surface area of contact between the strands would be twice that of single edge-to-edge contact which was believed to be a sufficient area of surface contact to produce a severable bond between the strands of adequate strength to prevent premature separation, yet not such a large single contact area, as in side to side contact, that would cause inseparable fusion. However, initial testing of the proposed new process did not produce the desired results, even when strand twisting was carefully avoided. It was found that when the two strands are joined along their projecting edges in the quench bath, the water becomes trapped between the strands in the area enclosed by the abutting edges and the oppositely facing recessed sides of the two strands. The trapped water is caused to boil by the heat of the molten strands, creating an increase in pressure between the two strands which prevents the strands from properly bonding together.
Thus, even though a very consistent, durable yet readily severable weld can be created between two conventional nylons strands to form a highly successful, tangle-resistant dual-strand monofilament cutting line, the same result have not heretofore been obtained with strands defining multiple raised cutting edges. The process disclosed herein achieves that result.